The main life-threatening substance is weed killer which very rapidly causes vomiting, diarrhoea, colic and persistent spasms and (sometimes only after agonizing days) lead to total respiratory paralysis.  

If you want to be sure to be able to help your dog quickly and efficiently in case of poisoning make sure to ALWAYS carry with you a syringe and a sufficient amount of atropine. Against the mentioned substances atropine is the most suitable antidote. You can buy it for a very low price in any pharmacy in vials of 1 ml. They should be stored under 25 degrees Celsius and so in summer you should frequently renew your stock. The syringes cost almost nothing and so costs are no excuse for anybody to not always have fresh vials handy for a case of emergency!

Dosage: up to 0,2 ml per kilo of body weight; that means a dog of 10 kilos should get about 2 ml (= 2 vials) of atropine. The atropine will wear off after about 6-8 hours and another injection might be necessary. Do call a veterinarian!

Injection (for the medically practised): a quarter of the dose intravenously (works fastest) and the rest intramuscularly and subcutaneously.

Injection (for amateurs): only intramuscularly, preferably into the buttock, deep enough to take the needle into the muscle but without going through it. If you are not confident that you are able to use that method, then use the subcutaneous method: grab the skin in the back of the neck of the dog so it forms a roll parallel to the dog’s body. Take the syringe body between two fingers and insert it on the bottom of the roll of skin and parallel to it. Make sure the needle does not exit. Now press the syringe. It helps to practise that last method on a stuffed animal so you are prepared when this happens to you and your dog for the first time.